This invention relates to control circuits and, in particular, to providing a ramp signal for slope compensating a control circuit.
A slope compensator is typically used to generate a voltage ramp waveform in order to stabilize a current mode switching regulator. Further, most, if not all, current mode regulators that operate above a 50% duty cycle require slope compensation to prevent subharmonic oscillation, as is known. A slope compensator operates by adding a linear ramp signal to a current feedback ramp signal. Typically, this is accomplished by creating an offset voltage at one input of a pulse width modulator (PWM) comparator. The offset voltage typically starts at 0 volts at the beginning of a clock cycle and linearly ramps up to a predetermined voltage during the on-time of the clock. However, the function of the slope compensator is typically not included within a chip of an integrated control circuit. Thus, users must add this function externally by using discrete circuitry.
Micro Linear manufactures some integrated circuits which include slope compensation within the integrated circuit itself. Some examples are: ML4809, ML4810, ML4811 and ML4812. However, these integrated circuits simply provide a level shift and buffer circuit similar to methods that use external discrete circuits. Further, these integrated circuits also require a dedicated programming pin for slope compensation.
Hence, a need exists for an improved integrated control circuit having slope compensation while not requiring the use of a dedicated pin.